Radio receiving system



Gct. 8, 1946. H. MAGNUsKl RADIO RECEIVING SYSTEM Filed July 3, 1943 2 Sheets-Sheet l ATTORNEY MUTING OSCILLATOR 8a REC 25 H. MAGNUSKI RADIO RECEIVING SYSTEM Filed July 3, 1945 2 Sheets-Sheet 2 ATTORNEY y Patented Det. 8, 1946 UNITED STATES PATENT oEFicE RADIO RECEIVING SYSTEM Henry Magnuski, Chicago, Ill., assignor to Galvin oration, Chicago, Ill., a corporation of Illinois Application July 3, 1943, Serial No. 493,322

(Cl. Z50-20) Manufacturing Corp 16 Claims.

The present invention relates to radio receiving systems and more particularly to improved methods and apparatus for blocking or muting a radio receiver to prevent the reproduction of noise voltages appearing in the system during periods when the system is conditioned for operation but a desired signal is not being received.

One of the problems involved in the design of a satisfactory radio receiving system is .that of quieting the system against the reproduction of noise voltages appearing in the receiving channel during periods of no signal reception. Such noise voltages may be produced as a result of thermal agitation or shot eects within the tubes provided in the system, by physical shock to the circuit elements of the system, or they may be picked up from extraneous noise voltage sources by the antenna-ground circuit of .the system. The frequencies of noise voltages thus introduced into a radio receiving system fall indiscriminately within the audio and super-audible frequency ranges, and regardless of the origin thereofthe audio components of such voltages may be reproduced at the loud speaker end of the system as objectionable noises. vIn those receivers which are used as communication equipment for emergency, police or military communication service, in particular, the receiving channels are normally maintained tuned to a single frequency and are normally conditioned for operation, so that the receiver will always pick up and reproduce at any time, signals of the selected frequency which may be radiated from a transmitting point either continuously or intermittently. In such receivers the reproduction of noise voltages of the character mentioned is annoying at best, and becomes unbearable in many instances. Various arrangements have been devised and'used t0 minimize the reproduction of such noise voltages in radio receiving systems. In thev usual arrangement, however, the magnitude of either a signal or noise voltage introduced into a radio vreceiver is the determining factor in the blocking and unblocking of the receiver. In other words, when the signal or the noise voltage introduced into the receiving channel exceeds a predetermined value, the channel is unblocked so that the noise or signal voltage is passed through the channel for reproduction by the loud speaker provided at the end thereof. 'I'he undesired opening of the receiver channel as a result of noise voltages appearing therein is particularly noticeable in areas of high noise level such as areas of heavy trafiic or zones in which other sources of electrical interference are present. If it is attempted to so adjust one of the prior art arrangements that noise voltages will not have the eifect of opening the receiver channel and hence render the audio section of the channel operative, then the receiver is not sensitive and is rendered incapable of reproducing received signals of loW signal carrier intensity.

In the copending application Serial No. 386,- 989 of Daniel E. Noble, filed April 5, 1941, and assigned to the same assignee as the present invention, an improved arrangement for muting the audio section of a receiver is disclosed and claimed, wherein the problems referred to above are wholly obviated. In this improved arrangement, the muting or squelch apparatus responds differently to noise voltages and to received signal voltages. More speciiically, the arrangement is such that the audio section of the receiving channel is blocked with increasing effectiveness the greater the magnitude of noise voltages appearing therein. A received signal carrier is utilized in the system to decrease the noise voltage components by an amount sufficient to permit the audio section of the receiving channel to be opened. This arrangement has the important advantage that its operation and effectiveness are not reduced or lost when noise voltagesV of large magnitude appear in the receiver in which it is provided. While the system of muting disclosed in the above referred to copending application is highly satisfactory in operation, the arrangement of the biasing circuits incorporated therein is Such that one or more electron discharge tubes of the indirectly heated cathode type must be used therein. Although such tubes are not objectionable in broadcast receivers, they do require an appreciable amount of time and current to heat the cathodes of the tubes after the energizing circuits therefor are completed. Accordingly, -they are not ordinarily usedin police and military receivers. When, however, it is attempted tc use tubes ofthey filamentary cathode type in the Noble arrangement, it is difficult to obtain .the bias voltages which are required in the blocking of the audio section of the receiver during no-signal periods.

It is an object of the present invention, therefor-e, to provide in a radio receiving system, an improved arrangement, utilizing tubes of the iilamentary cathode type, for positively rendering the system nonresponsive to noise voltages appearing therein, regardless of the magnitude thereof( Y It is another object of the invention to provide an improved muting or-squelch arrangement of 3 the character described which is particularly adapted for use in a radio receiver of the frequency modulated type.

It is a further object of the invention to provide an improved method for preventing noise voltages appearing in a radio receiving system from being passed through the system to operate a signal responsive device.

According to another object of the invention, an oscillator, which is controlled by noise and signal voltages appearing in the receiver, is utilized selectively to control the audio section of the receiver so that noise voltages appearing in vthe receiver are not translated.

In accordance with still another object of the invention, the muting or squelch apparatus associated with the receiver is provided with an electron discharge tube having a screen electrode, and provisions are made for automatically controlling the operating potential applied to this electrode so that the system is rendered inactive when a desired signal is not being received and is rendered active when a desired signal is received.

According to a still further object of the invention, noise voltages appearing in the receiving channel of the system in the absence of a received signal are used to control the potential applied tothe screen electrode of the oscillator tube so that the noise signals are not reproduced, and a received desired signal is utilized to change this potential in the required sense to render the system active.

It is still another obect of the present invention to provide improved muting apparatus of the character described which is so arranged that it will operate in areas of either high noise level or low noise level and yet when operating in an area of high noise level will not prevent the reception of a weak signal, all without adjustment of the system or any of the parts thereof.

More generally stated, it is an object of the present invention to provide a squelch or muting system of the character described which is simple in arrangement, positive and reliable in operation, does not materially reduce the sensitivity of the associated receiving system, and cannot be falsely operated in response to lnoise voltages of large magnitude appearing in the system.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

Figs. 1 and 2, when laid end to end in the o-rder named, illustrate a frequency modulation radio receiving system having incorporated therein improved muting or squelch apparatus characterized by the features of the invention briefly referred to above;

Fig, 3 is a graph illustrating the noise response characteristic of the receiver shown in Figs. 1 and 2; and

Fig. 4 illustrates a modified arrangement of certain parts of the system shown in Figs. 1 and 2.

Referring now more particularly to Figs. 1 and 2 of the drawings, there is illustrated, in partially schematic form, a frequency modulated radio receiving system having combined therewith improved muting or squelch apparatus characterized by the features o-f the present invention. Briefly considered, the system comprises an antenna-ground circuit I0, a tunable antenna circuit II, a tunable radio frequency amplifier I2,

a tunable first mixer stage I3, a first interrnediate frequency amplifier I4, a fixed tuned second mixer stage I5, a second intermediate frequency amplifier I6, first and second limiter stages I1 and I8, respectively, a frequency discriminator or signal detecting device I9, an audio frequency amplifier 2D, and a loud speaker 2 i, all connected in cascade in the order named. As indicated by the dash line U, the tuning elements of the three tunable stages II, I2 and i3 are ganged together for unicontrol operation in an entirely conventional manner. 1n general, the improved muting or squelch apparatus comprises a high pass filter network A22 coupled to the output side of the discriminator I3, a noise amplifier and rectifier 23, a direct current amplifier 24 and a muting oscillator and rectifier 25. These stages are likewise connected in tandem in the order named and respond to noise voltages appearing in the receiving channel to impress a blocking bias voltage upon the audio frequency amplifier 20 in the manner explained below.

More specifically to consider certain of the elements provided in the receiver, it will be noted that the discriminator I9 is capacitance coupled through the coupling condenser 29 to the output circuit of the second limiter stage I8. This discriminator comprises a resonant circuit 26, a pair of diode rectifier tubes I9a and ISb, the space current paths of which are respectively shunted by load resistors 31| and 32, a radio frequency bypass condenser 33 having substantially a negligible impedance to frequencies of the order of the second intermediate frequency, and a stabilizing or unbalancing condenser 30 connected in shunt with the load resistor 32 and the space current path through the diode Ib. The resonant circuit 26 comprises a pair of series connected condensers 26h and 23e which are shunted by an inductance element 26a for tuning the circuit to the second intermediate frequency, i. e., the center frequency of a modulated carrier voltage appearing in the three stages I6, I1 and I8 during signal reception. Preferably, the last-mentioned element is of the variable permeability type, being provided with an adjustable powdered ferrous metal core, the position of which may be changed to alter the inductance of the element within the desired limits. The circuit constants of the resonant circuit 25 are so chosen that this circuit is provided with a band pass characteristic such that all desired signal components of a frequency modulated carrier appearing in the second intermediate frequency section I6, I1, I8 of the receiver may be detected and impressed upon the input circuit of the audio frequency amplifier 2B. As indicated above, the voltage appearing across the output side of the second limiter I8 is impressed upon the discriminating network I9 through a coupling condenser 29 which is connected at one side thereof to the junction point between the two condensers 26h and 23C. Audio frequency voltages detected through operation of the discriminator I9 appear across the condenser 33, and are impressed upon the input side of the audio frequency amplifier tube 2Ua through a coupling circuit which includes a radio frequency decoupling resistor 34, an audio frequency filter comprising the resistor 35 and condenser 36, an audio frequency coupling condenser 3l, and a voltage dividing network comprising the two resistors 38 and 40 and a direct current blocking condenser 4I. It will be understood in this regard that the proportion of the available audio frequency voltage appearing across the series connected resistors 38 and 40 which is impressed between the input electrodes of the amplifier tube a, and hence the volume of signal reproduction, is determined by the setting of the wiper 39 along the resistor 38.

As will be explained more fully below, noise voltages appearing in the signal transmission channel of the receiver in the absence of a received signal modulated carrier are passed through the discriminator I9 and appear as detected audio frequency and super-audio frequency voltages across the condenser 33. Such detected voltages are impressed across the high pass filter network 22 and those components thereof having frequencies above the cutoiT frequency of the lter network are impressed between the input electrodes of the tube 23a included in the noise amplifier and rectifier 23. More specifically considered, the high pass lter 22 comprises a pair of series condensers 41 and 48 and a pair of s shunt resistors 45 and 4B, and is designed to pass those components of noise voltages having frequencies above the normal signal reproducing band of the receiver. The noise amplifier section of the tube 23a works into a noise rectier circuit which comprises the diode section of the tube and a load resistor 52. This rectifier circuit is coupled to the anode of the tube 23a through a coupling condenser 54 which is of appropriate impedance to pass any noise currents which may be transmitted through the high pass filter 22. Anode and screen potentials are supplied to the tube 23a through the resistors 5U and 5I, the second of which is by-passed to ground through a condenser 53.

Rectiiled noise voltages appearing across the load resistor 52 are utilized to control the bias between the input electrodes of the tube 24a provided in the direct current amplier 24. The

initial or threshold bias established between the electrodes of this tube is derived from a voltage dividing network which comprises the series connected resistors 58a, 58o and 58o bridged across the available source of anode potential, and is provided with a tap 51a adjustable along the resistor 58D to impress a variable positive potential upon the control electrode of the tube 24a through the lter resistor 51. A filter network comprising the resistor and the condenser 56 is provided for preventing alternating components of the voltage appearing across the load resistor 52 from being impressed between the input electrodes of the tube 24a. Screen and anode potentials are supplied to the amplifier tube 24a through the resistor 60 and the resistors 6G and 59 in series, respectively.

The direct current amplifier 24 as controlled by the variable bias voltage derived from the load resistor 52, is utilized to control the starting and stopping of the muting oscillator and rectifier 25. This stage of the muting or squelch apparatus comprises a dual purpose tube 25a having an oscillator section which includes a tuned frequency determining circuit 33 connected between the output electrodes of the tube through a condenser S. The resonant circuit 63 is fixed tuned to a particular frequency of from 200 to 300 kilocycles and comprises an inductance element 63a shunted by a tuning condenser 53h. It is regeneratively coupled to the input electrodes of the tube 25a by means of a feed-back circuit which comprises an inductance element 68 inductively coupled to the inductance element 63a and connected in series with a parallel connected grid leak resistor 61 and condenser 66 between the control grid and cathode of the tube 25a. Anode potential is supplied to the tube 25a over a path which includes the inductance element 63a and a blocking resistor S5, this resistor being by-passed to ground by a condenser B4. The oscillator section of the tube 25a is coupled to the rectifying circuit of the tube through a coupling condenser 62, and the indicated rectifying circuit serially includes the diode rectifier section of the tube, a resistor 69 and the encircuited portion of a voltage dividing resistor 1I. Any bias voltage appearing across the load resistor S9 and the enoircuited portion of the resistor 'II during operation of the oscillator and rectifier stage 25 is negatively applied to the control grid of the audio frequency ampliiier tube 20a over a path which comprises the resistor 6I, the resistor 4e and the lower lencircuited portion of the resistor 3B. In this regard it is pointed out that the resistor 'II is shunted by a source of bias potential 7S and is provided with a tap 'l2 adjustable therealong for the purpose of impressing a normal operating bias voltage between the input electrodes of the tube Zila during periods when the muting oscillator and rectifier stage 25 is inactive. The biasing arrangement comprising the current source "Hl, the voltage dividing resistor YI and the adjustable wiper 12 is illustrated for explanatory purposes only, itbeing noted that the normal operating bias for the audio amplifier tube 20a may conveniently be derived from a stage of the receiver preceding the frequency discriminator I9. More specifically, this bias voltage is preferably derived from the control grid biasing network utilized at the second mixer stage I5 of the receiving channel.

In order to insure that the system will be speedily conditioned for operation when cathode heating current issupplied to the cathodes of the various tubes provided in the system, all of the tubes, With the exception of the discriminator diode Ida are of the lamentary cathode type. The excepted diode rectifier I 9a must of necessity be of the indirectly heated cathode type since the cathode thereof is, during operation of the discriminator I9, maintained at potentials substantially above the reference ground potential present upon the iilamentary cathodes of the remaining tubes provided in the system. In this regard it is noted that, preferably, the two diodes ISa and ISb are of the commercial types 1A3 and 1S5, respectively, the noise amplifier and rectifier tube 23a and the muting oscillator and rectifier tube 25a are commercial type 1S5 pentodes, and the direct current amplifier tube 24a is a commercial type 1M pentode.

Briefly to consider the operation of the system, it will be understood that when cathode heating current is supplied to the cathodes of the various tubes provided therein, these cathodes are immediately heated to their normal electron emitting temperatures, at which time the system is fully conditioned for operation. Due to the lamentary character of the energized cathodes, they are rapidly heated to electron emitting temperatures following the energization thereof. Assuming that the system is thus conditioned for signal reception and that the tunable stages of the system are appropriately tuned to the center frequency of a desired frequency modulated carrier, the signal carrier voltage appearing across the antenna-ground circuit Il) is transmitted through the tunable antenna circuit II to the input side of the tunable radio frequency ampliiier I2. This voltage as amplified by the amplifier l2, is mixed with the carrier output of the local oscillator provided in the rst mixer stage I3 to be converted into aI signal modulated intermediate frequency carrier which is selected and amplified in the first intermediate frequency amplifier I4, and is impressedacross the input circuit of thev second mixer stage l5. In the second mixer stage, the intermediate frequency carrier output from the amplifier |4- is mixed with the carrier frequency produced through operation of the local oscillator provided at this mixer stage, so that a beat frequency carrier, modulated with the signal voltage and of the desired second intermediate frequency, appears at the output side of the second mixer stage. This modulated carrier is ampliiied and transmitted successively through the second intermediateV frequency amplifier I6 and the limiter stages l1 and I8 to the input side of the discriminator le. In this discriminator, the modulation components of the second intermediate frequency carrier, as represented by deviations? inl the carrier frequency from the established center frequency, are detected and impressed' across the voltage dividing network comprising the resistors 38 and 4Q through the carrier frequency decoupling resistor 34-v and the audio frequency coupling condenser 31. The portionl of this voltage which appears between the wiper 39 and' ground, is impressed between the input electrodesof the audio frequency ampliiier tube l'la, amplified by this tube, and transmitted through the coupling transformer 43' to the loudspeaker 2|Y for reproduction in the usual manner.

Referring now more particularly to the operationiof the discriminator I9, it will be noted that tliiscrciut isr essentially a four terminal bridge circuit having two arms which respectively include-'the'ccndensers 25h' and 25o of equal capacitances.l .et third" arm of the bridge comprises the ceqoacitive impedancel of the diode i301.. The fourth.' armor.' the bridge comprises the combined capacitive impedances of the diode i279 and the condenser S. The inductance element 26a is thusbridged between' two terminals of: the bridge circuit', and the frequency modulated signal-voltu age' is-appiied to the circuit across the other two terminals thereof'. Since the load resistors 3i and 32? have impedances,. at the frequencies involved,`.which are far inV excess of the capacitive impeds'rn'cesv of' the diode legs of the bridge circuit,V they may be neglected in analyzing the circuit. Again,.the capacitance of the condenser 33.1 is so much greater than that of either diode ieg of the circuit, that this condenser may also be neglected in an analysis of the circuit. In the bridgecircuit arrangement thus provided, the voltage appearingl at the output side of the discriminator` is the difference between the absolute voltages to ground at the upper and lower terminals of the inductance element 25a. More specifically considered, if the capacitance of the condenser 26h equalsthat of the condenser 26e, which it does, and the capacitance of the two diodelegsof the circuit are equal, such that the bridge is balanced, the currents respectively traversingf the condensers 251) and 26e are equal so that equal voltage drops appear across these condenscrs. Accordingly, no difference between the volt-agosto ground is developed at the upper and lower terminals of the inductance element 26a, regardless-of the frequency of the exciting voltage-applied to the circuit. In the actual circuit,

however,4 the capacitance of that leg which in- 7 when-- equal, no' direct voltage appears 60 exciting' frequency for the discriminator cludes the diode ISb is greater than thecapacitance of the leg including the diode [9a by an amount equal to the capacitance value of the condenser 30, such that the bridge is unbalanced.

5 Accordingly, during excitation of the circuit, the

current traversing the condenser 26e exceeds the current traversing the condenser 2Gb so that a current is caused to now through the inductance element 26a. The magnitude of thisv current l obviously depends upon the reactive impedance of the inductance element 26a at the-particular frequency of.V excitation, andthe direction of current flow is such that the voltage drop across the condenser 2Gb is enhanced and that across the l condenser 26o is decreased. It will be understood,

therefore, that by suitably proportioning the impedance ofy the inductance element a. relative to the reactive impedances of the condensers' 26h and 26e at a particular center frequency, thereby 20 to establish a given relationship between the currents traversingthe circuit elements Tic, 2Gb and 28o, the absolute voltages between the upper and lower terminals of the inductance element 28a. and ground become equal. In their relationship g5 to-eachother, however, these voltages are out of phase, so that a differencevoltage actually exists between the7 upper and lower terminalsv of the inductance element 26a, This difference voltage of course equal= to the Vector sum of the abso- 30 luteY voltages between the upper and lower terminalsof the inductance element 26a and ground. The particular frequency at which these absolute voltages become equal to balance the bridge, represents the center frequency at which the voltage appearing at the output side of the discriminator between the cathode of the diode lilal andA ground, becomes zero. In this regard it is pointed out that when the bridge is balanced, so that the voltages from the upper and lower terminals of the inductance element 26a. tol ground are equal,

equal direct voltages are producedl acrossthe load resistors 3|- and 32; These direct voltages are opposingly combined in a'I direct current path through the inductance element 26a, so that between the cathode ofthe diode 19a and ground.

Asfthe exciting voltage' for the discriminator I'S is increased above the center frequency, due to the signalA modulation thereof at an audio rate,

ther reactive impedances ofthe circuit constants changel to alter the relative magnitudes of the currents traversing the circuit elements' 26a, 2Gb and' 26o so that the voltage fromfthe upper terminalof the inductance element 26a to ground exceeds that'between' the4 lower terminal of the inductance element 26a and ground. Accordingly, avoltagewhich is positive with respect to ground is produced between the cathode of the diode I'Sa' and ground; If, on the other hand, the I9 is decreased below thecenter frequency, the reactive'impedances ofthel circuit constants change to'alter the relative magnitudesofthe currents traversing the circuit elements 26a, 26b'and 26e sothat the-voltage between the lower terminal ofthe inductance element 26a andl ground exceeds that between the upper terminal of the inductance element 26aY and ground. Asa result, an output voltage which is negative with respect to ground is produced between' the cathode of the diode I9aand ground; It has been found that the extent-or magnitude ofthe discriminator output voltage varies linearly in accordance with the` departure of the exciting frequency from the 5 centerY intermediate frequency to which the discriminator network I9 is center tuned. It will be understood, therefore, that if the frequency of the carrier appearing at the output side of the limiter I8 is frequency modulated in accordance with a given audio signal a corresponding audio frequency voltage is accurately reproduced across the condenser S3 at the output side of the discriminator i9.

To consider somewhat more fully the action of the condenser 30 in stabilizing the operation of the discriminator network I9, it may be pointed out that if the impedances of the four legs of the bridge circuit are perfectly balanced, changes in the exciting frequency will not produce the desired differences of potential between the upper and lower terminals of the inductance element 26a and ground. By providing the condenser 3l) connected in the manner illustrated, however, thereby to insurev that the over-all capacitance between the lower terminal of the inductance element ao and ground exceeds that between the upper terminal of this inductance element and ground, the desired circulating current within the resonant circuit 26 will always be produced to insure stability of circuit operation.

Operation of the muting apparatus As will be apparent from the foregoing eX- planation, when the system is conditioned for signal reception but is not receiving a desired signal,

noise signal voltages appearing in those stages of the receiving channel which precede the discriminator i9 appear as audio and superaudible frequency voltages at the output side of this discriminator. More specifically, these voltages are transmitted through the intermediate frequency and mixer stages of the receiver, and are detected by the discriminator I9 to appear as audio frequency and super-audible frequency voltages at the output side of the discriminator. Such noise voltages may be produced as a result of thermal agitation within the tubes provided in the receiver, shot effects, extraneous noise voltages appearing across the antenna-ground circuit I or by physical shock to the circuit elements provided in the receiver. Regardless of the origin thereof, however, the noise signals are manifested as audio and super-audible frequency voltages appearing between the cathode of the diode rectifier lila and ground at the output side of the discriminator i9, and in the absence of the described muting apparatus would be passed through the audio frequency amplifier 2li to the loud speaker 2| for reproduction.

More specifically considered, the noise response of the receiver is graphically illustrated in Fig. 3 of the drawings wherein the noise voltage appearing between the cathode of the diode ISa and ground is plotted as a function of the selected signal carrier input voltage appearing across the antenna-ground circuit lil. From a consideration of this curve it will be noted that when no signal carrier is being received, the noise voltage apnearing at the output side of the discriminator I9 is high, and that the magnitude of this voltage is sharply reduced in response to the application of a selected signal carrier to the antenna-ground circuit I0. The decrease in the level of the noise voltage which accompanies the transmission of a selected signal through the receiver is largely eected in the amplitude limiters I'I and I8.

To consider the action of the muting apparatus, it is pointed cut that the noise voltage appearing between the cathode of the diode I9a and ground at the output side of the discriminator I9 is im- 22. This filter acts tc pass only those components of the noise voltage having frequencies above the normal signal reproducing band of the receiver. For example, this filter may be designed to pass frequencies above kilocycles. The noise voltage appearing across the output side of the lter 22 is impressed between the input electrodes of the noise amplifier and rectier tube 23a and appears in amplified form across the coupling condenser Eil and the diode section of the tube 23a. Due to the rectifying action of the diode section of this tube, a direct voltagev is produced across the load resistor 52 which varies in magnitude in accordance with the magnitude of the noise voltage impressed between the input electrodes of the tube. This direct voltage, i. e., that across the resistor 52, is negatively applied to the control grid of the direct current amplifier tube 2da through the resistor 55 in opposition to the fixed bias voltage normally positively applied to the control grid of the tube 24a through the resistor 5l. The negative voltage appearing across the resistor 532 so greatly predominates over that positively applied to the control grid of the tube Zilla that this tube is biased beyond its space current cutoff point. Accordingly, the voltage drops across the two resistors 66 and 59 are sharply decreased to very low values, with the result that the full voltage 0f the available source of anode current is positively applied to the screen electrode of the oscillator and rectier tube 25a. The application of this voltage to the screen electrode of the tube 25a initiates the operation of the oscillator section of this tube, so that an oscillatory voltage is developed across the series connected coupling condenser 62 and the space current path between the diode electrodes of the tube. Due to the action of the diode section of the tube 25a in rectifying the oscillatory voltage, a direct bias voltage is produced across the diode load circuit comprising the resistor 89 and the encirouited portion of the resistor '12. This bias voltage is negatively applied to the control grid of the audio frequency amplifier tube 20a over a path which includes the resistor 5I the resistor 40 and the encircuited portion of the resistor 38. The magnitude of this bias voltage is sufcient to bias the tube 20a beyond cutoff, whereby the noise signals are prevented from being transmitted through the audio frequency amplifier 20 to the loud speaker 2I for reproduction.

As will be apparent from further consideration of the curve shown in Fig. 3 of the drawings, when a selected signal carrier of substantial magnitude appears across the antennaground circuit I0, the limiters Il and I8, together with the discriminator I9, function sharply to decrease the noise voltage developed at the output side of the discriminator. This produces a corresponding decrease in the bias voltage developed across the load resistor 52. When the negative bias applied to the control grid of the tube 24a. is thus reduced to a low value, the current flow through the resistors Si! and 59 and the space current path of the tube 24a is sharply increased to produce a corresponding increase in the voltage drops across the two identified resistors. As a result, the voltage which is positively applied to the screen electrode of the oscillator and rectifier tube 25a through the two resistors 60 and 59 is sharply decreased to a value such that operation of the oscillator section of this tube cannot continue. When the production of an oscillatory voltage across the space the oscillator 'and rectifier tive bias voltage applied band of the receiver,

giriamo 11 current path of the tube the negative bias voltage developed across the rectifier load circuit comprising the resistor 69 'and the encircuited portion of the resistor 'H in the manner just explained is reduced to zero, permitting the normal negative bias voltage appearing between the adjustable tap T2 and ground to be impressed upon the control electrode of the audio frequency amplifier tube 2da. When this amplifier tube is thus unblocked or biased to a normal value, the audio section of the receiver is rendered operative to amplify the signal frequency components of the received signal and to transmit the same to the loud speakerZl for reproduction.

From the foregoing explanation it will be understood that normally, i.e., when the system is conditioned'for signal reception, the noise signals appearing in the receiver are utilized to completely Vblo'cl: the audio section of the receiver against the transmission of noise signals to the loud speaker 2 l. 'More specifically, the component circuit elements of the muting apparatus should be so chosen that inthe absence of a desired signal the negative bias voltage developed at the upper terminal of the resistor/GI is approximately 'volts. To this end, from 40 to 50 volts must be positively applied to the screen electrode of tube '25a when a tube of the commercial 155 type is employed in the oscillator and rectifier stage 25. Further, the

a is thus arrested,

'component circuit elements of the muting apparatus 'should be such that Awhen a selected frequency modulated carrier is received having a `magnitude exceeding a predetermined low value, the voltage positively applied to the screen electrode of the tube 25a is dropped Vto approximately 20 Volts such that operation of the oscillator Ysection of the tube is arrested. In the absence of an oscillatory voltage between the 'anode and cathode of this tube, the only negato the'control grid of the audio frequency amplifier tube Zila is that developed across that portion of the resistor 'Il `which is connected between the adjustable tap 12 and ground, which voltage is of the order1 of 'a'fewvolts When the apparatus is designed to have the characteristics just described, the audio channel 'of the receiver will at Vall times remain blocked during periods when a selected signal is not being received an'd'will 'be automatically unblocked when a selected signal istransmitted through. the receiver to the discriminator 2l Vfor detectio'n. In this regard it will 'be understoodthat since the high pass filter'22 will not pass frequencies within the`normal signalreproducing the muting apparatus is not responsive to the audio frequency components of a received signal carrier and thus this apparatus is prevented from blocking the audio section of the receiver against the transmission 'of detected signal voltages to the loud speakerZl. It will also be understood that theV magnitude of the voltage developed across the load resistor 52 in the noise amplifier and rectifier stage 23 is directly proportional to the magnitude of the noise voltage components which are passed through the high pass filter 22' and impressed between the input electrodes of the tube 23a. Accordingly, the greater the magnitude of the noise voltages appearing in the receiver, the greater the bias voltage impressed between the cathode and 'control grid of the direct current amplifier tube 24a. 4Once thisV tube is 'biased beyond cut off, the voltage drops across the two resistors 59 and 60 are unaffected by changes in the noise level and the Vpositive potential applied to the screen electrode of the muting oscillator and rectifier' tube 25a remains constant. Accordingly, the bias voltage negatively applied to the control grid of the audio frequency amplifier tube 26a remains constant with increases in the'magnitude of the noise Voltage appearing at the output side of the discriminator I9 above a predetermined value. Thus it is apparent that in the novel muting or squelch apparatus disclosed above, the inherent disadvantages of those prior art systems which operate strictly upon a noise or signal voltage amplitude basis is wholly obviated. It will also be apparent from the preceding explanation that no tubes of the indirectly heated cathode type are required in the muting apparatus. Thus, by providing the oscillator and rectifier stage `25, operative in the manner described above, it is possible to utilize a direct current amplifier '24 having a filamentary type cathode which is maintained at the reference ground potential. Accordingly, the muting apparatus is immediately rendered operative to perform its intended function when cathode heating current is supplied to the cathodes of the tubes vprovided therein. This novel feature of the arrangement is of particular utility in portable or fixed position units which are to be utilized in police, military or other emergency work.

As Vindicated above, the greater the noise intensity, the greater the negative vvoltage applied to the tube `2da from the resistor 52 and the farther beyond cut oi this tube is biased. This of course means that in areas of high noise level a greater signal strength is required to open the audio channel than would be required in an area of low noiselevel. Thus the difficulty of adjust- Ving the threshold bias to a value suitable for universally correct operation under widely varying noise level conditions to insure reception of both strong and relatively weak signals becomes readily apparent. The ideal arrangement is that in which the voltage across the load resistor 52 will remain constant with increasing noise intensity after it has been increased to a value just sufficientto bias the tube 24a beyond cut 01T.

In order to obviate the difficulties referred to in the preceding paragraph, the modified arrangement illustrated in Fig. 4 of the drawings maybe employed. In this arrangement, reference characters corresponding to those used in Figs. 1 and 2 have been employed for the purpose of'identifying corresponding portions of the system. In fact, the four stages i8, I9, 22 and 23 as shown in Fig. 4 may be incorporated in the system of Figs. 1 and 2 as a unit. From an examination of the modif-led arrangement of Fig. 4, it will be noted that a biasing link has been provided between the biasing circuit for the control grd of the second limiter tube I8a and the control grid of the noise amplifier and rectifier tube 23a. In this regard it is noted that the second limiter i8 is of entirely conventional design, such, for example, as that illustrated in Fig. of August Hunds book entitled Frequency VModulation, rst edition, published in 1942 by 13 Vents audio or radio frequency feed-back from the output side of the high pass lter 22 to the input side of the limiter I8. A condenser B is also provided for isolating the biasing circuit just described from ground through the iter resistor 45.

In considering the operation of the arrangement shown in Fig. 4 ol the drawings, it will be understood that a voltage which is negative with respect to ground is developed across the resistor 16 when signal or noise voltages are applied to the input side or" the second limiter I8 from the preceding limiter Il. The magnitude of this voltage varies directly in accordance with the noise or signal intensity and reaches a maximum when the second limiter becomes saturated. Thus, the cathode-control grid biasing circuit of the second limiter functions as a source of negative bias potential during oil-signal periods, the extent or magnitude of the negative bias appearing across the resistor 16 varying in proportion to the intensity of the noise voltages appearing in the system. A portion of this voltage, i. e., that appearing between the wiper lla and ground, is negatively applied to the control grid of the noise amplifier and rectiliei tube 23d through the resistor 78. Thus, an arrangement is provided for varying the noise voltage gain through the tube 23a inversely in accordance with the noise level as measured at the second limiter stage I8.

In other words, when the noise voltages appearing at the output side of the high pass lter 22 are at a maximum and thus tend to produce the maximum rectified voltage across the resistor 52, the bias voltage negatively applied to the control grid of the tube 23a from the second limiter stage I8 is also at a maximum with the result that the gain or amplication of the noise voltages through the tube 23d is a minimum. As the noise in the system decreases, the voltage appearing at the output side of the filter 22 decreases in like manner and this decrease is accompanied by a corresponding decrease in the negative bias voltage applied to the control grid of the tube 23a, from the second limiter stage I8. Accordingly, the gain through the tube 23a. is increased to prevent a large decrease in the voltage appearing across the load resistor 52. It will thus be apparent that by providing the biasing link between the second limiter i8 and the noise amplifier and rectifier 23, the tendency of the voltage across the load resistor 52 to increase with an increase in the noise appearing at the output side of the filter 22 is substantially reduced. Conversely, the tendency of the voltage across the resistor 52 to decrease sharply with a sharp decrease in the noise voltage appearing at the output side of the lter 22 is also minimized through the action of the described biasing circuit. Thus by properly proportioning the values of the circuit constants involved, the voltage appearing across the load resistor 52 may be held substantially constant over a relatively wide range of noise intensities.

When a signal is received, the two limiter stages I l and I8 and the discriminator stage I9 operate in the exact manner explained above to produce a sharp decrease in the noise voltage appearing at the output side of the filter 22. Thus, the voltage across the resistor 521s dropped to a value such that operation of the muting oscillator and rectifier 25 is arrested. It will be understood from the above explanation that the biasing link between the second limiter I8 and kthe noise amplier and rectifier 23, by holding the rectied noise voltage which appears across the resistor 52 substantially constant, prevents the tube 24a from being biased far beyond cut oi Accordingly, relatively weak signals may be received in such an area. The extent of the limiting action as provided by the added biasing link is of course determined by the setting of the wiper Ila along the resistor i6. This wiper, in itsl relationship to tli`e setting of the potentiometer wiper along the resistor 53h, should be so set that the muting oscillator and rectifier 25 will be operated when the system is used in areas having a wide range of noise level intensities and yet the operation thereof will be arrested when a signal of suicient amplitude for discernible reproduction is received. In practice the proper settings for the two potentiometer wipers may easily be ascertained by manual adjustment of the two wipers in different zones of the particular area where the system is to be used.

While different embodiments of the invention have been described, it will be understod that various modifications may be made therein, which are within the true spirit and scope of the invention.

I claim:

1. In a wave signal receiving system which includes a signal responsive device, means responsive solely to undesired noise signals appearing in said system in the absence of a received desired signal for developing a control voltage, means responsive to a received desired signal in the presence oi" noise signals for changing said voltage, an oscillator, means responsive to said voltage for controlling an electrical characteristic of the output of said oscillator, and means controlled by said oscillator output for determining the responsiveness of said device.

2. In a radio receiving system which includes a signal responsive device, means responsive to undesired noise signals appearing in said system for developing a control voltage, means responsive to a received desired signal in the presence of noise signals for effecting a decrease in said voltage from one value to a lower value, an oscillator controlled by said voltage and operative to produce an oscillatory voltage only when said control voltage exceeds said lower value, and means controlled by said oscillatory voltage for preventing said device from responding to noise signals appearing in lsaid system in the absence of a received desired signal.

3. In a radio receiving system which includes a signal responsive device, means responsive to undesired noise signals appearing in said system for developing a control voltage, means for preventing said last-named means from responding to a received desired signal, means responsive to a received desired signal in the presence of noise signals for changing said voltage, an oscillator, means responsive to said voltage for changing an electrical characteristic of the output of said oscillator, and means controlled by said oscillator output for preventing said device from responding to noise signals appearing in said system in the absence of a received desired signal.

4. In a radio receiving system which includes a signal responsive device, means responsive to undesired noise signals appearing in said system for developing a control voltage, means for preventing said last-named means from responding to a received desired signal, means responsive to a received desired signal in the presence of noise signals for effecting a decrease in said voltage from one value to a lower value, an oscillator controlled by-said voltage and operative to produce an oscillatory voltage `only when said .control voltage exceeds said lower value, and means controlled by said oscillatory voltage for preventing said device from responding to noise signals appearing in said system in the absence of a received desired signal.

5. In a radio receiving system which includes a signal transmission cha'nel 'and va signal responsive deviceVan oscillator, means responsive to the appearance of undesired noise signals in said channel in the absence of Va received desired signal for causing the operation of said oscillator, means controlled by the oscillatory voltage developed by said oscillator for blocking said channel against the transmission of signals to said device, and means responsive to a desired signal received in the presence of noise signals for decreasing the control exercised by said first named means until said oscillator stops oscillating, thereby to render said device responsive to the desired signal.

6. In a radio receiving system which includes a signal transmission channel and a signal responsive device, means responsive to the appearance of undesired noise signals in said channel in the absence of a received desired signal for developing a control voltage, means responsive to a desired signal received in the presence of noise signals for decreasing said control voltage below a predetermined value, an oscillator including an electron discharge tube provided with a screen electrode and operative to produce an oscillatory voltage only when a voltage exceeding said predetermined value is applied to said screen electrode, means for impressing the control voltage developed by said rst named means upon said screen electrode, and means-controlled by .said oscillatory voltage for preventing said device from responding to noise signals appearing in said channel in the absence of a received desired signal.

'7. In a frequency modulated radio receiving system which includes a signal responsive device preceded by a signal carrier transmission channel having a limiter provided therein, means coupled to said channel at a point following said limiter and responsive to the appearance of -undesired noise signals in said channel in the labsence of a received frequency modulated carrier for developing a control voltage, means'including said limiter for effecting a decrease in said control voltage below a predetermined value in response tothe reception of a frequency modulated carrier in the presence of noise signals, an oscillator controlled by said voltage to operate-only when said control voltage exceeds said vpredetermined value, and means Ycontrolled by the output of said oscillator for preventing said device from responding vto noise signals appearing in said channel.

8. In a frequency modulated radio receiving system which includes a signal responsive device preceded by .a signal transmission cha-miel comprising a limiter and a frequency discriminator connected in tandem in the order named, means coupled to said channel at a point following said frequency discriminator and responsive to the appearance of undesired noise signals in said channel in the absence of a received frequency modulated carrier for developinga control voltage, means including said limiterand said frequency discriminator for effecting a .decrease in said control voltage belowa predetermined value Vinresponse to the reception .of afrequency. modulated carrier in the presence of noise signals, an oscillator controlled by said voltage to Aoperate only when said voltage exceeds said predetermined valuey and means controlled by the output of said oscillator for preventing said device from responding to noise signals appearing in vsaid channel.

9. In a frequency modulated radio receiving system which includes a signal responsive device preceded by a signal transmission channel comprising a limiter and a frequency discriminator connected in tandem in the order named, means coupled to said channel at a pointfollowing said frequency discriminator and responsive to the appearance of undesired noise signals in said channel in the absence of a received frequency modulated carrier for developing a control voltage, means including said limiter and saidfrequency discriminator for effecting a decrease in said control voltage below a predetermined value in response to the reception of a frequency modulated carrier in the presence of noise signals, an oscillator including an electron discharge tube provided with a screen electrode and operative only when a voltage exceeding said predetermined value is applied to said screen electrode, means for impressing the control voltage developed by Said rst named means upon said screen electrode, and means controlled by the output of said oscillator for preventing said device from responding to noise signals appearing in said channel.

l0. In a radio receiving system which includes a signal responsive device, an oscillator which includes an electron discharge tube provided-with a screen electrode and having an output controlled in accordance with the potential applied to said screen electrode, means for applying a potential to said screen electrode, means controlled by noise voltages appearing in said system for controlling the potential applied to said `screen electrode to change said oscillator output,

and means controlled by said oscillator output for preventing said device from responding to noise voltages appearing in said system.

11. In a radio receiving system which includes a signal responsive device, an oscillator including an electron discharge tube provided with a screen electrode and operative to produce an oscillatory voltage only when a control voltage exceeding a predetermined value is impressed upon said screen electrode, means controlled by noise signals appearing in said system for impressing a control voltage exceeding said predetermined value upon said screen electrode, and meanscontrolled by said oscillatory voltage for preventing said device from responding to noise signals appearing in said system.

12. In a radio receiving system which includes a signal responsive device, an oscillator including an electron discharge tube provided with a screen electrode and operative to produce an oscillatory voltage only when a control voltage exceeding a predetermined value is impressed upon said screen electrode, means controlled by noisesignals appearing in said system for impressing a Acontrol voltage exceeding said lpredetermined value upon said screen electrode, means for .preventing said last named vmeans from responding to av received signal, and means controlled by said oscillatory voltage for preventing said device from responding to noise signals appearing in said system.

13. A radio receiving system comprising a sigpnal .responsive device, a signal transmission channel for transmitting signals to said device, a plurality of electron discharge tubes coupled to said channel in cascade and having cathodes of the filamentary type connected to operate at substantially the same potential, and means including said tubes operative in response to undesired noise signals appearing in said channel in the absence of a received desired signal for preventing said device from responding to the noise signals.

' 14. A radio receiving system comprising a signal responsive device, a signal transmission channel for transmitting signals to said device, a plurality of electron discharge tubes coupled to said channel in cascade and having cathodes of the lamentary type connected to operate at substantially the same potential, means including said tubes operative in response to undesired noise signals appearing in said channel in the absence of a received desired signal for blocking said channel against the transmission of said noise signals to said device, and means included in said channel and responsive t a desired signal received in the presence of noise signals for controlling said last-named means to unblock said channel.

15. In a radio receiving system which includes a signal responsive device, means for selecting noise appearing in said system in the absence of a received signal, means controlled by the selected noise for developing a voltage which tends to increase with increasing noise intensity, means controlled by said voltage for preventing said device from responding to noise appearing in said system so long as said voltage exceeds a predetermined value, means for adiustably varying the predetermined value of said Voltage at which said last-named means is rendered operative to prevent said device from responding to the noise, and additional means for controlling said second-named means to variably decrease the tendency of said voltage to increase with increasing noise intensity.

16. In a radio receiving system, means for selecting noise signals appearing in said system in the absence of a received signal which have frequencies diierent from the frequencies of received desired signals, means responsive to the selected noise signals for developing a control voltage, means including an oscillator responsive to said voltage for preventing signal or noise translation in said system, and means responsive to a received desired signal in the presence of noise signals for effecting a predetermined decrease in said control voltage, thereby to render said oscillator non-responsive to said control voltage and thus render said last-named means ineffective to prevent signal translation in said system.

HENRY MAGNUSKI. 

